We’ve recently had several questions from the public asking whether this year has been particularly windy compared to others and if there’s any explanation for this. There’s lots of ways at looking at these questions, but the quick answer from our National Climate Information Centre is that – yes, it has been windy this year and a lack of high pressure seems to be to blame. Here Mike Kendon, climate information scientist at the Met Office, takes a detailed look at the questions.

How many calm days?

One way of looking at this is to consider how many days there have been which have not been windy – i.e. calm days – and how this compares with the historical record. The bar chart below counts the number of days each year, for the UK overall, where at least 20 weather stations have recorded a maximum gust speed of 10 Knots (11 mph) or less. This is equivalent to, at most, a gentle breeze, while 20 stations would indicate such conditions fairly widespread for at least 24 hours.

2015 thus far has seen only 8 such days; this being the fewest number of calm days across the UK for at least 20 years – but bearing in mind this covers less than 7 of 12 months of the year so far. However, more notably none of these days have fallen in May, June or July so far.

Chart showing the number of days per year where at least 20 UK weather stations have recorded a maximum gust speed <= 10 Kt (11 mph). 2015 data up to 22 July.

Pressure patterns

Calm days are typically associated with areas of high pressure, which normally bring dry, settled conditions during summer and cold, frosty conditions in winter – but common to both seasons often light winds. Areas of high pressure tend to block the prevailing westerly airflow across the UK. However, the variability of our climate means that some years see more days of high pressure, others see fewer such days.

The first map below shows the mean sea level pressure relative to average across the North Atlantic for the period January to June 2015. Over this 6-month period the pressure has been lower than normal to the north of Scotland but higher than normal to the south-west, resulting in a predominant westerly airflow over the UK, meaning that our weather has often been windy. Although during 2015 there have been some periods of high pressure, for example during March and early April, they have been relatively infrequent, particularly from May onwards. The pressure difference shown on the map between Iceland and the Azores is known as the North Atlantic Oscillation (NAO) index.

Pressure anomaly (difference from 1981-2010 average) in mb for the period January to June 2015 and January to June 2011, based on NCEP / NCAR Reanalysis data. Image provided by the NOAA/ESRL Physical Sciences Division.

Rainfall patterns

A westerly airflow across the UK is generally associated with low-pressure systems from the Atlantic bringing windy conditions and rain-bearing fronts. Since this is the UK’s prevailing wind direction, the north-west is, on average, much wetter than the south-east, being most exposed to this direction. In addition, rainfall here is further increased due to the effect of hills and mountains.

During 2015, the persistent westerly airflow has resulted in this north-west / south-east contrast in rainfall patterns being exaggerated. For example, Achnagart, a weather station in the West Highlands of Scotland recorded 2082mm of rain in the period from 1st January to 22 July 2015, compared to 237mm for the same period at St James’s Park, Central London – 9 times as much.

The map below shows rainfall totals compared to average from January to June 2015. So, this rainfall pattern is consistent with this westerly weather type, absence of prolonged spells of high pressure, and relatively windy nature of 2015 so far.

Rainfall January to June 2015 as % of 1981-2010 average for that period.

3 Responses to Has 2015 really been that windy?

As someone who participated in some of the Met Office’s long-range-forecast conferences back in the late 60s and early 70s, the pressure anomaly depicted above comes as no surprise. The SST anomalies south of the Grand Banks indicated a more zonal flavour than normal with a strong mid-Atlantic high in the first half of the period with NW flow over UK. The pattern changed to favour more of a straight westerly later. It’s a shame that the work by H H Lamb et al seems to have been forgotten – except by old fogies like myself.

There is a way using the objective Lamb Weather Types from the CRU to examine the windiness across the British Isles. As far as I see the gale index does support what Mike Kendon says about 2015 so far. I can’t agree that the cause is less anticyclonic weather this year though. You can find graphs and tabulated data to support my conclusions here: